Primary combustion units such as fireplaces, wood burning stoves, or wood or coal burning furnaces have many advantages for use in remote locations or in aesthetic accouterments in dwelling houses, but they suffer the disadvantage that they can lead to serious pollution and environmental problems. For example, wood burning fireplaces, although aesthetically pleasing in many respects, are associated with severe heat loss problems, much of the heat goes "up the chimney" and more importantly, serious emissions problems are associated with the release of pollutants into the atmosphere. Typically, heating units such as stoves and fireplaces release substantial quantities of carbon monoxide, hydrocarbons and other organic pollutants and particulate materials such as ash and partially combusted cinders into the atmosphere.
Various systems have been proposed to reduce the quantitative pollutants released into the atmosphere, many of which involve the use of some sort of secondary afterburner system in which conversion of pollutants to less harmful products, e.g. carbon monoxide to carbon dioxide occur through oxidation, direct pyrolysis or catalytic conversion over simple catalyst materials. For example, U.S. Pat. No. 5,007,404 to Hall et al discloses a secondary combustion chamber for a wood stove in which preheated secondary combustion air is forced into the secondary chamber in the flue passage from a wood stove. The Hall system includes a forced air fan which blows air over a heating element in response to the measured temperature of combustion gases flowing from the primary combustion chamber through an opening into the secondary combustion chamber. Glow plugs are located in the secondary combustion chamber and a control circuit activates ignitor circuits for the glow plug. The fan is temperature controlled, for example, it may be turned on at about 700.degree. F. and turned off at about 1200.degree. F.
U.S. Pat. No. 4,422,437 to Hirschey discloses a system incorporating a fire box which functions as a primary combustion chamber. A horizontal baffle is interposed between the primary combustion chamber and a secondary chamber in which a forward afterburner chamber is separated from a rearward flue gas and by-pass chamber by means of intermediate partitions. Here, fresh air is supplied over catalytic grid type combusters and mixed with heated unburned flue gases to enable combustion of the flue gases as they move past the catalytic combuster tubes.
U.S. Pat. No. 4,434,782 to Traeger discloses a wood burning furnace in which hot combustion products from the fire box flow through a restricted fire box collar in an indirect heat exchange relationship with a heat exchanger. The heat exchanger includes electrically powered heating coils over which air is forced by a blower.
U.S. Pat. No. 5,263,471 to Shimek et al discloses a wood burning combustion chamber in which temperature is raised in the upper portion to pyrolytic temperature conditions which substantially consumes volatile combustible materials as they flow through a relatively narrow exhaust passageway into a chimney stack. Refractory insulation material is heated to pyrolytic temperatures so that radiant heat is transmitted through a glass door thus heating remote areas of the dwelling unit or other structure to be heated. Yet another system for disposing of creosote products is disclosed in U.S. Pat. No. 4,425,305. Here, catalytic material is coated on heating tubes through which combustion products pass from a stove into a chimney stack.